Primarily designed and developed for navigation by the U.S. Department of Defense in the late 70's, GPS has revolutionized the positional data collection techniques not only in surveying and mapping but also in numerous other areas. One such area is an automatic vehicle location system ("AVLS") for use in a fleet management system, where locations of fleet vehicles are tracked by a base station for both real-time and post-processed systems.
GPS generally provides real-time positional information as to the location of a vehicle equipped with a GPS receiver. However, for tracking locations of fleet vehicles in a fleet management environment, only the relative position of each vehicle unit to a fixed base station need be determined for post-processed vehicle data base management. One such system is the Fleetmaster.TM. system available from Rockwell International, Newport Beach, Calif., the assignee of the present application.
FIG. 1 illustrates a typical DGPS system, where a base station antenna 10 has a fixed, or surveyed, location for observing GPS satellites 15 using a geodetic GPS receiver. Note that the "position" in GPS realm means geodetic latitude, geodetic longitude and geodetic height. The base station, through the GPS receiver, then calculates error parameters in each satellite range. The correction parameters are broadcast to all mobile units 19 for all satellite ranges. Upon receiving the correction parameters, the mobile units 19 apply the correction parameters to derive their accurate position.
With DGPS, accuracy can be improved to within 10 m or less, e.g. the length of a bus or truck. Therefore, it is able to provide enough accuracy to effectively bypass the Selective Availability ("SA") restrictions intentionally imposed by the U.S. government. However, to implement DGPS, a typical implementation would require the base station to transmit an RTCM ("Radio Technical Commission for Maritime Services")-104 message to the fleet for the fleet vehicles to update their individual positions. As will be explained further, this proves to be an inefficient use of the communications channel.
For fleet management systems such as an AVLS, GPS-equipped vehicles allow the base station to determine where the fleet vehicles are located. In AVLS, each GPS receiver aboard the vehicle acquires position, velocity and heading, i.e. "locations", information, as well as a host of others, and transmits the information back to the base station through a communications link such as radio or cellular connection. In the meantime, the base station determines the error information as to each satellite and broadcasts the information to each fleet vehicle through an RTCM-104 message.
The problem of this conventional AVLS is that in a fleet environment, the forwarding message by the base station after receiving, calculating and packaging information consumes quite a bit of the communications channel, thus making this paradigm less cost effective. Because the base station must send all the differential correction data to all fleet receivers, this message forwarding creates a costly burden upon the system.
Another conventional DGPS base station is illustrated in more detail in FIG. 2. A base station antenna 20 is set to acquire pseudo-ranges ("S.sub.1 . . . S.sub.n ") from the earth orbiting satellites. The pseudo-ranges S.sub.1 . . . S.sub.n are received through a base station GPS receiver 21 such that they can be processed by a data processor 22. Note that the GPS receiver 21 should be equipped with enough channels ("N channels") to accommodate all the GPS satellites available. Concurrently, an antenna position vector P, which represents the already known antenna position, is input to the data processor 22 for processing. Also, a vector E.sub.n (t) representing the position of the n-th satellite relative to the center of the earth is input to the data processor 22 for determining a vector R.sub.n for the n-th satellite, which represents the vector range between the n-th satellite and the base station antenna position. Note that the vector E.sub.n (t), i.e. coordinates of the n-th satellite at time "t" as it is broadcast by the n-th satellite, is obtained from an ephemeris file 28 derived from each n-th satellite data message in earth-centered coordinates as shown in the vector chart 29. It is to be noted that in the present application, the notation for a vector is indicated in bold styles.
The data processor 22 obtains the DGPS corrections for the n-th satellite according to the following computation:
E.sub.n -P=R.sub.n, and E*.sub.n =R.sub.n *, since P*=0 (Note: "*" denotes rate of change in time, or the "derivative", of the variable) and PA1 abs (R.sub.n)-abs (S.sub.n)=abs (PRC).sub.n, and PA1 abs (R*.sub.n)-abs (S.sub.n *)=abs (RRC).sub.n =DGPS corrections for n-th satellite.
The DGPS corrections as determined by abs (PRC).sub.n ("pseudo-range correction") and abs (RRC).sub.n ("pseudo-range rate correction") can then be formatted by a data formatter 25 according to the RTCM-104 protocols. Thereafter, the RCTM-104 message is modulated by a communications link 26 before it is transmitted by a data link transmitter 27 to the remote fleet receivers. It should be noted that a presumption has been made in this implementation that only "n" satellites are in view of any one base station and the number "n" should be fully accommodated by the N-channel receiver at the bast station, i.e. n.ltoreq.N. As can be understood by those skilled in the art, the communications link 26 and data link transmitter 27 can easily be overburdened by the RTCM-104 message, which is broadcast to all remote units by the base station.
FIG. 3 illustrates a DGPS base station configuration with active forward RTCM-104 transmissions. The DGPS corrections obtained by the data processor 35 are formatted by the data formatter 34 so they can be modulated by a modem 32 for transmission. The modulated data are transmitted in RTCM-104 data stream 30, which identifies the DGPS corrections associated with all observable satellites, e.g. SV.sub.1, SV.sub.2, SV.sub.3, SV.sub.4, . . . SV.sub.n, by a communications link 31, such as SMR ("Specialized Mobile Radio") or cellular, to a fleet vehicle 36. The DGPS corrections are processed by the vehicle's GPS receiver to obtain corrected GPS data 37 and reported back to the modem 32 through the communications link 31.
Those skilled in the art can readily appreciate that while this system achieves DGPS accuracy, a high data rate is required in base station transmissions to prevent DGPS data latency. Also, there is less reporting time available for fleet vehicles, although air time usage is significantly increased.
Therefore, it is desirable to obtain accurate DGPS corrections for the fleet vehicles for the fleet operator at the base station for fleet management purposes.
Also, it is desirable to use the DGPS corrections to obtain corrected DGPS positions for the base station without incurring the data transmission cost.
Further, it is desirable to obtain DGPS positions without the burden of having to broadcast a forward message to link all the fleet vehicles.
Further, it is desirable to obtain DGPS accuracy without significantly altering the existing communications protocol, while taking advantage of the existing communications protocol.